Generally, stents are used as an alternative to surgery to obtain and maintain the patency of a variety of body passageways, while maintaining the integrity of the passageway. The environments of human vasculature and body passageways are characterized by varied, dynamic, and mobile anatomy. Vessels vary from simple to complex shapes, can be uniform in diameter or change abruptly or gradually from one diameter to another, and are subjected to a range of internal forces exerted by blood or air pressure, and external forces exerted by an assortment of anatomical structures surrounding and adjacent to these body passageways. It is critical that stents be designed to accommodate significant variation in the shape and size of body passageways while providing structural support and flexibility as required by particular indications of use.
The primary role of a stent is to provide radial expansion and scaffolding within the affected segment, thereby improving patency or flow and preserving the viability and full function of distal tissues. In performing this primary function, however, a stent must exist in harmony with surrounding structures of the body, including vessels, nerves, muscles, organs, and other tissues. Each region of the anatomy presents a unique combination of loads, interactions, and constraints that will be experienced by the implant. In many regions of the anatomy, these boundary conditions will vary not only with location, but also with time. These temporal variations, including motions associated with the cardiac pulsatile cycle, gait cycle, respiratory cycle, or other dynamic events, are especially important considerations for the durability of the implant itself, as well as the efficacy of the therapy. Consequently, stent designs are needed that can (1) provide adequate outward radial support to remodel the lumen and improve distal perfusion in the case of vascular anatomies, (2) provide adequate crush recoverability when subjected to compression by the surrounding muscles or external forces, (3) provide adequate flexibility to accommodate localized stretching, compression, bending, or torsion in mobile segments of the artery or other passageway, (4) provide durability to survive the motions associated with the cardiac, respiratory or gait cycles and/or limb flexion, and (5) provide uniform scaffolding throughout the treatment region, including the local regions adjacent to calcification that may be subjected to highly focal cyclic loading or displacement. These competing demands have proven difficult to resolve with a single design.